This episode begins a long line of Defiant nits. The Dominion is this massive threat which could come pouring out of the wormhole at any moment. The Defiant was assigned to DS9 specifically to counter that threat. Yet Sisko treats it like his own personal taxi, jaunting off to Trill so Dax can get treated and leaving the station undefended. Why not take a Runabout? That's exactly what they are for!

Plotline

When Dax begins to suffer dramatic mood swings and hallucinations, Sisko returns with her to Trill so that their medical experts can diagnose the problem. But as her condition worsens, Sisko uncovers startling evidence that Dax is more than he thought - and the revelation may threaten the entire basis of Trill society.